1. Field of the Invention
The invention relates to a semiconductor device, and more particularly to a recess gate type transistor, which increases saturation current, and a method for fabricating the same.
2. Description of the Related Art
As semiconductor devices are highly integrated, widths of gates are gradually decreased and formation of high-density junctions and channels is required. However, intervals of channels are decreased and electric fields applied to sources and drains are gradually increased. The increased electric fields accelerate electrons between the sources and the drains and generate hot carriers, which damage to the surfaces of the gates, thus causing degradation of the semiconductor device.
Particularly, in cells, since the increase of the electric fields generates a leakage current, it is difficult to refresh the cells. Accordingly, in order to improve refresh characteristics, a recess gate type transistor is used instead of a planar gate type transistor having gate electrodes formed on the plan surface of a substrate. Such a recess gate type transistor is structured in that a gate insulating film is formed on side walls and bottom surfaces of recesses formed in a substrate and the recesses are filled with a conductive film, such as a polysilicon film. The recess gate type transistor improves refresh characteristics and reduces the generation of hot carriers and the generation of punch through, but still has an increased length of channels.
When the channel length is increased, a saturation current (Idsat) decreases, thus causing difficulty in fabricating a high-speed semiconductor device.
In order to fabricate the recess gate type transistor, ion implantation for adjusting threshold voltage (Vt) is performed. However, in this case, the adjustment of the threshold voltage (Vt) may be limited. In order to solve the problem, in the ion implantation, ions are implanted at a designated tilt angle. However, it is difficult to implant ions at a designated tilt angle due to a shadow effect of recessed areas. Accordingly, general recess gates cannot use all channels.
FIG. 1 is a schematic, sectional view of a conventional recess gate type transistor.
With reference to FIG. 1, recesses 11 formed in a semiconductor substrate 10 have a general shape. Here, in order to adjust threshold voltage, ions are implanted into the recesses 11 at a right angle. When ions are implanted into the recesses 11 at the right angle, the ions are implanted into the bottom surfaces of the recesses 11, but are not implanted into the side surfaces of the recesses 11. Here, reference numeral 12 represents an anti-etching film.
Here, in order to adjust the threshold voltage, ions are implanted into the recesses 11 at a designated tilt angle. However, in this case, ion implanted areas of the side walls of the recesses 11 are still limited due to shadow effect. That is, sides of the recesses 11, in which a source or a drain is formed, are covered with a mask, and then ion implantation is performed so that a punch through margin only at the sides of the recesses 11 is assured. This method requires an additional step, thereby causing time and economic losses.
Accordingly, a semiconductor device for improving the above characteristics and a method for fabricating the same are required.